This invention relates to a reset mechanism of a counter for counting the number of revolutions of a photoconductive drum provided on a printer for forming visible images on a recording sheet, more particularly, to a reset mechanism capable of resetting the counter only when a new photoconductive drum is installed on the printer.
Laser printers are recently coming into wide use and compact inexpensive printers utilizing semiconductor lasers in particular are increasingly being put to practical use. A laster printer of that sort has been so contrived as to obtain hard copies of image data on a recording sheet by means of the so-called electrophotographic duplicating processes including exposing process for exposing a photoconductive drum charged with electricity to a laser beam modulated according to image data on graphs as well as characters and subsequently passing the resulting latent images through a developing process for forming a visible image on the surface of the photoconductive drum. A transferring process for transferring the visible image to a recording sheet, a fixing process for fixing the transferred image on the recording sheet by pressurizing the recording sheet with the heat, and the like.
In such a laser printer constructed as an electrophotographic imaging device, the photoconductive drum wears out through in small increments as it undergoes each step of the electrophotographic duplicating processes, e.g., transfer or cleaning of the surface of the photoconductive drum. If the photoconductive drum has become worn, it will become poorly charged or allow the presence of an after-image on its surface. This will also cause bad setting, i.e. inadequate transfer to recording sheet, etc.
Consequently, the photoconductive drum is formed into a unit so that it can simply be replaced and equipped with a counter for counting pulse signals which are synchronized with revolution of the photoconductive drum to use the count for determining the timing at which the photoconductive drum is to be replaced.
In other words, the photoconductive drum is replaced at the point of time that the count indicated by the counter has reached a predetermined value, i.e., a predetermined number of recording sheets have been printed. The counter is cleared when the photoconductive drum is replaced. There is provided a microswitch, for instance, for detecting the presence or absence of the photoconductive drum and the counter is reset using presence-absence-presence signals applied from by the microswitch when the photoconductive drum is replaced.
However, since the power supply of the printer is turned off when the photoconductive drum is replaced, the microswitch will not be actuated and consequently the counter cannot be reset. In other words, the counter will advance to the existing count prior to the replacement and start counting again. The counter will otherwise be reset despite the face that the photoconductive drum is reinstalled after it is removed once for maintenance, inspection and the like.
A known arrangement to meet the aforementioned problems is to provide the photoconductive drum unit with a pin abutting against the microswitch for detecting the presence or absence of the photoconductive drum in such a manner as to make the pin break immediately after it has come in contact with the microswitch simultaneously when the unit including the photoconductive drum is installed. Since the pin is broken at the time the unit is initially mounted, the counter is prevented from being reset when the photoconductive drum is reinstalled after it has been once removed.
However, the counter is not reset while the power supply of the printer itself it turned off as in the preceding case where such a pin is not employed; the problem, in this case, remains i.e. a mounting of a new photoconductive drum is undetected.